The obese gene in human, rat and mouse encodes a protein hormone having an open reading frame 167 amino acid residues in length, called leptin, also known as OB protein or the ob gene product. Removal of the signal sequence yields a mature secreted 16 kilodalton protein that is 146 amino acid residues in length.
OB protein is produced primarily by adipocytes of white adipose tissue (WAT). OB protein is secreted directly into the extracellular space and travels through the blood stream. OB protein affects the cells of its target organs by binding to the OB receptor protein, OB-R, that is found on the extracellular surface of the plasma membrane of target cells. Binding of OB protein to OB-R activates the intracellular second messenger cascade of the JAK-STAT system, which is characteristic of activation of cytokine type I receptors.
OB protein is produced in adipocytes in proportion to the mass of stored fat, thereby providing a hormone signal for a lipostatic feedback circuit, which is mediated by the OB receptor. While OB proteins of different species show a close similarity in their sequences, the sequences of OB proteins are not closely similar to other types of proteins. For example, the human ob gene sequence and its mouse homologue (85% sequence identity) have been reported to have no sequence similarity to other proteins of known structure (DiFrancesco, V., et al., Protein Topology Recognition from Secondary Structure Sequences: Application of the Hidden Markov Models to Alpha Class Proteins, J. Mol. Biol. 267: 446–463 (1997) at page 457).
Although OB protein is composed of a single peptide chain, an intrachain disulfide bond between cysteine 96 and cysteine 146 is required both to stabilize the conformation of the molecule and to confer in vivo biological activity (Rock, F. L., et al., The Leptin Haemopoietic Cytokine Fold is Stabilized by an Intrachain Disulfide Bond, Horm. Metab. Res. 28: 649–652 (1996)). It is believed that the special geometry of the A and D major helices must be maintained in order to dock to a conserved receptor trough in the receptor molecule, a requirement that produces structural similarity between OB proteins and cytokines in the face of negligible sequence conservation (Id. at 651.).
One accepted and successful animal model of human obesity is the genetically obese mouse bearing the recessive obese mutation (ob/ob). The mouse model reproduces not only the human obesity condition, but also develops non-insulin dependent diabetes mellitus (NIDDM, also known as type II diabetes mellitus). Homologous obese genes have been described in mouse, rat and human.
The mouse is also a widely accepted and successful model of sepsis, septic shock and systemic inflammatory response syndrome (SIRS), a term which describes the clinical syndrome of sepsis without regard to its cause. Simple models, involving a large bolus dose of lipopolysaccharide (LPS) administered to mice and using mortality as the primary outcome variable, are well suited for preliminary pharmacological studies of new drugs or other therapeutic agents (Fink, M. P. & Heard, S. O., Laboratory Models of Sepsis and Septic Shock, J. Sure. Res. 49: 186–196, 1990, at 188–189).
Both the ob gene (Zhang, Y., et a, Positional cloning of the mouse obese gene and its human homologue, Nature 372: 425–432 (1994); accession No. U18812, SEQ ID NO. 1) and its receptor (Tartaglia, L. A., et al., Identification and expression cloning of a Leptin Receptor, Cell 83: 1263–1271 (1995); Chen, H., et al., Evidence that the Diabetes Gene Encodes the Leptin Receptor: Identification of a Mutation in the Leptin Receptor Gene in db/db Mice Cell 84: 491–495 (1996); accession No. U46135, SEQ ID NO. 2) have been cloned. Shorter versions of the OB receptor, termed the OB-Ra, OB-Rc and OB-Re forms, are produced by alternative splicing of the OB-R mRNA (Lee, G. H., et al. Nature 379: 632–635 (1996)). The full length OB receptor is called the OB-Rb form.
Neural activity in specific regions of the central nervous system (CNS), such as the hypothalamus, controls processes related to food intake and energy expenditure. The cloning of the OB protein gene and the OB receptor gene and the localization of OB receptor expression in the hypothalamus has provided supporting evidence for this view as well as suggesting possible mechanisms for relating food intake to stored fat reserves. The OB protein is produced by adipocytes in proportion to the mass of stored fat and, hence, it acts as the signal to a lipostat control circuit. This lipostat signal is transduced at the target cells by the OB receptor, OB-R, in the CNS, resulting changes in neural activity that regulate both food intake and metabolic rate.
Metabolic derangement is an important characteristic of the host response to critical illness called the acute phase response that characterizes conditions such as sepsis and septic shock (Kushner, I. Ann. N.Y. Acad. Sci. 389: 39–48 (1982)). Hypothermia is a metabolic response that may be pertinent clinical prognostic factor in systemic inflammatory response syndrome in humans (Brivet, F., et al. Crit. Care Med. 22: 533–534 (1994)).
There is a need for at least one disease marker for systemic inflammatory response syndrome (SIRS) and related conditions. Hereinafter, the term SIRS is used to denote sepsis, septic shock, sepsis syndrome, and related conditions. Disease markers have numerous functions. In this case, a marker for SIRS would be useful for predicting the development of SIRS, identifying patients with SIRS, predicting outcome, aiding timing and targeting of therapeutic interventions, and determining the pathogenesis of SIRS in patients (Parsons, P. E. & Moss, M. Early Detection and Markers of Sepsis, Clinics in Chest Medicine 17:199–212 (1996)).